Propylene oxide is an important article of commerce finding use in diverse areas. For example, polymerization with alcohols as initiators affords polyether polyols. Polymerization catalyzed by, e.g., ferric chloride affords poly(propylene oxide) polymers with molecular weights of 100,000 or more. Reaction with water gives a spectrum of propylene glycols, including monopropylene glycol, dipropylene glycol, tripropylene glycol, and so forth. Reaction with ammonia, or amines generally, affords aminoalcohols. Each of the foregoing are important niche commodities, either per se or as reactive components in, e.g., polyurethane manufacture.
The principal route to propylene oxide employs the so-called chlorohydrin process where propylene is first convened to its chlorohydrin and the latter is subsequently dehydrochlorinated to produce the epoxide. Chlorohydrin formation is effected by reaction of aqueous chlorine or hypochlorous acid with propylene under acidic conditions. Dehydrochlorination is accomplished by treating the chlorohydrin with a base.
More recently the oxidation of propylene with peroxide has become competitive to the traditional chlorohydrin route. In addition to hydrogen peroxide, organic peroxides may be used as the oxidant and include materials such as t-butyl hydroperoxide, t-pentyl hydroperoxide, ethylbenzene hydroperoxide, cumene hydroperoxide and peracetic acid. Where organic peroxides are used an organic byproduct necessarily is formed. For example, if t-butyl hydroperoxide is used as the oxidant t-butyl alcohol is an unavoidable byproduct and any process using this organic hydroperoxide must either find t-butyl alcohol as an economically acceptable byproduct, (i.e., its market must make the overall process economically viable) or the process must provide for recycling t-butyl alcohol to t-butyl hydroperoxide. Epoxidation catalysts generally are soluble metal compounds of, e.g., molybdenum, vanadium, tungsten, and titanium.
Although the use of hydrogen peroxide as the oxidant would be singularly advantageous, since its byproduct is water whose recycle is unnecessary, until recently commercial routes to propylene oxide have focused on the use of organic peroxides, such as t-butyl hydroperoxide and ethylbenzene hydroperoxide, and commercially successful processes based thereon have been developed. Even though the use of hydrogen peroxide as an oxidant has many attractions, one limitation is the practical need for having a hydrogen peroxide generation facility at the site, i.e., it is not commercially feasible to transport large amounts of dilute hydrogen peroxide to the oxidation site and the transport of concentrated solutions presents safety hazards. Commercial generation of hydrogen peroxide is merely the reaction of hydrogen and oxygen, although performed indirectly rather than directly. Therefore, hydrogen peroxide generation requires a source of hydrogen. Consequently, if one wishes to oxidize propylene with hydrogen peroxide it is necessary that the complex have a continual and guaranteed on-site source of hydrogen. We have recognized that the foregoing conditions can be met if propylene is formed with the coproduction of hydrogen, and that an integrated process can be devised making propylene oxide production via peroxide-based oxidation of propylene a commercial reality. The remainder of this application is devoted to a description of our invention.